1. Field of the Invention
The present invention relates to the digital measurement of voltage signals for protective relaying purposes in situations where the voltage is derived from, a capacitive voltage transformer (CVT), and particularly to improving the dynamic accuracy of protective functions that are dependant on voltage measurement.
2. Description of Background
Protective relays are devices that are designed to identify and isolate failures in a power system. A failure often takes the form of an insulation breakdown, (fault) and results in a change in a system voltage and/or current. Typically, within a power system, protective relays are configured to detect failures of a specific portion of the power system, theses specific portions of the power system being commonly referred to as zone. A protective relay guarding against short circuits should never respond to events other than failures within its particular zone. Further, the longer a fault persists in the power system, the greater the likelihood that the stability of the entire power system will be compromised. For this reason, a protective relay should be designed and configured to correctly identity faults that have occurred within its zone in a minimum possible time period.
Conventionally, microprocessor-based protection devices for power systems operate by sampling input currents and/or voltages at regular time intervals, digitally extracting selected properties of those signals (e.g., magnitudes, angles, time derivatives, harmonies, etc.), and comparing the signal properties in reference to one another or in reference to a threshold. Instrument transformers are used to isolate a protective relay from the power system and to reduce a signal to a level wherein the signal can be processed by the electronic circuitry of the relay. In high voltage and extra high voltage power systems, CVTs are often used to reduce a voltage from a range of hundreds-of-thousands of volts (a primary voltage level) to tens-of-volts (a secondary voltage level) before supplying the voltage signals to a protective relay. Typically, CVTs are cheaper than magnetic voltage transformers but create problems for protective relays since they have a tendency to add specific transients to the voltage signals as they are transformed to the secondary level.
CVT generated transients tend to have relatively significant magnitudes and long durations. This aspect is particularly important for protecting transmission lines where the Source Impedance Ratio (SIR—the ratio between the system equivalent impedance and the relay reach impedance) is large. The primary voltage during a line fault is very low under a large SIR. The voltage signal is crucial for proper operation of the distance relay but becomes significantly distorted by transient components that are not present in the power system but are generated by the CVT.
Generally, the CVT transient will have a decaying DC component bin may also have a decaying oscillatory component(s). When the well-known Fourier algorithm is applied to voltage measurement (which is typical in microprocessor-based relays) the magnitude may be significantly underestimated due to the CVT transient. This will impact the fault identification performance of the relay. Protective relays typically incorporate a mechanism that can deal with CVT transients to some extent. Known methods include the introduction of an adaptive delay or the dynamic reduction of the fault detection zone of the relay.
Another method of dealing with the CVT transient is to insert a filter into the voltage signal path that is an inverted representation of the CVT transfer function. This removes the distortion generated by the CVT, thus resulting in a signal that is an accurate reproduction of the power system voltage. This method performs optimally only when the filter coefficients reflect the parameters of the particular CVT that is connected to the relay.
Presently, there is a need for a self-tuning mechanism that allows application of the CVT correcting filter without a prior knowledge of the parameters of the corrected CVT, and also allows constant adjustment of the properties of the filter in order to follow changes in the CVT.